Yrast isomers in
95
Ag,
95
Pd, and
94
Pd
N. Ma
˘
rginean,
1,2
D. Bucurescu,
2
C. Rossi Alvarez,
3
C. A. Ur,
2,3
L. D. Skouras,
4
L. P. Johnstone,
5
D. Bazzacco,
3
S. Lunardi,
3
G. de Angelis,
1
M. Axiotis,
1
E. Farnea,
3
A. Gadea,
1
M. Ionescu-Bujor,
2
A. Iorda
˘
chescu,
2
W. Krolas,
6,8
Th. Kro
¨
ll,
1
S. M. Lenzi,
3
T. Martinez,
1
R. Ma
˘
rginean,
2
R. Menegazzo,
3
D. R. Napoli,
1
P. Pavan,
3
M. De Poli,
1
B. Quintana,
7
C. Rusu,
1,2
P. Spolaore,
1
and J. Wrzesinski
6
1
INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy
2
H. Hulubei National Institute for Physics and Nuclear Engineering, Bucharest, Romania
3
Dipartimento di Fisica dell’ Universita ´ and INFN, Sezione di Padova, Padova, Italy
4
Institute of Nuclear Physics, NCSR Demokritos, Aghia Paraskevi, Greece
5
Department of Physics, Queen’s University, Kingston, Ontario, Canada K7L 3N6
6
Institute of Nuclear Physics, Krakow, Poland
7
Grupo de Fı ´sica Nuclear, Universidad de Salamanca, Spain
8
Joint Institute for Heavy Ion Research, Oak Ridge, Tennessee
Received 7 January 2003; published 10 June 2003
The yrast level schemes of three neutron deficient nuclei with mass A 95 have been studied with the
reaction
58
Ni+
40
Ca at 135 MeV, using the GASP -ray array, the ISIS silicon ball, and the n-ring neutron
detector. Excited levels, including a (23/2
+
) spin-gap isomer, are reported for the first time in the heaviest
N =Z +1 nucleus experimentally investigated,
95
Ag. In
95
Pd, the yrast line above the ground state has been
observed and connected to states above the known isomeric level 21/2
+
, showing for the first time that this is
indeed a spin-gap isomer. In
94
Pd, the 95.6-keV isomeric transition was confirmed, and its E 2 character was
firmly established. The experimental observations are compared with current shell-model calculations.
DOI: 10.1103/PhysRevC.67.061301 PACS numbers: 21.10.-k, 21.60.Cs, 23.20.Lv, 27.60.+j
New experimental information about neutron deficient nu-
clei with mass above 90 is of considerable interest for shell-
model calculations aimed at the description of nuclei close to
100
Sn. Being close to the N =Z line, they are expected to
show enhanced neutron-proton pairing effects based on the
T =0,I =1,I
max
configuration 1. The nuclear structure in
this region is mainly given by ( p
1/2
, g
9/2
) hole configurations
in the doubly magic
100
Sn, and therefore the neutron-proton
pairing is dominated by the g
9/2
neutron and proton orbitals.
Older shell-model calculations of Gross and Frenkel 2 in
the proton ( p
1/2
, g
9/2
) subshells have been performed assum-
ing a semimagic
88
Sr core and a set of two-body matrix
elements, which describe well many experimental data.
Other sets of two-body effective interactions were deduced
for this region more recently, but most of them are tuned for
nuclei close to the stability line. Going towards heavier
neutron deficient nuclei, a better specification of the effective
interaction becomes crucial. Experimental information con-
cerning nuclei close to the N =Z line is not so rich as for the
isotopes close to stability. In many cases, such as the rp-
process calculations, physics based on the properties of pro-
ton rich nuclei relies on shell-model estimates instead of un-
available experimental quantities. Thus it is very important
to have reliable shell-model description for the N Z nuclei
of this region. One of their characteristics that must be de-
scribed is the occurrence of high-spin isomeric states.
In
95
Pd, a high-spin isomeric state with a half-life of 14 s
decaying by both
+
/EC where EC stands for electron cap-
ture and -delayed proton emission has been discovered,
assigned as (21/2
+
), and placed at approximately 2 MeV
excitation energy 3,4. Shell-model calculations of Ogawa
5 have predicted that the first 21/2
+
state in this nucleus
has lower excitation energy than the first 15/2
+
and 17/2
+
states. This inversion would create a spin-gap isomer, since
the 21/2
+
state could be deexcited only by decay compet-
ing with E 4 decay to the first 13/2
+
state. The calculated
position of this state relative to the 17/2
+
state was shown to
depend rather critically on the value of the two-body matrix
element g
9/2
2
| V
pn
| g
9/2
2
J =9
5. Recent -ray spectroscopic
investigations have assigned excited states above the iso-
meric level, but its real position has still remained unknown,
as the excited states above the ground state could not be
observed 6. In
94
Pd, seven yrast transitions were identified
following the decay of a 0.53(1) s half-life of an isomeric
state 7,8, which has been associated to the 14
+
isomer
predicted by shell-model calculations. In a very recent study
of
94
Ag decay 9, some new rays were assigned to
94
Pd
and placed below and above the isomeric state. Consistent
information about the level structure of the N =Z +1 nucleus
95
Ag was not available in literature until the present work.
Only very recently three rays have been attributed to this
nucleus, but they could not be placed in a level scheme 10.
Shell-model calculations 5,11 predicted another possible
spin-gap isomer in this nucleus. The present experimental
data enrich significantly the knowledge of this nuclear region
and allow a much better test for the current shell-model cal-
culations in nuclei close to N =Z =50.
The
94
Pd,
95
Pd, and
95
Ag nuclei have been populated in
the
58
Ni+
40
Ca reaction performed at the Legnaro XTU Tan-
dem accelerator with a
40
Ca beam of energy 135 MeV. The
incident beam energy favored the two- and three-particle
evaporation channels. The beam intensity during the experi-
ment was about 8 particle nA, and a
58
Ni foil of thickness
6 mg/cm
2
was used as target.
The rays were detected with the GASP detector array
12 arranged in its standard configuration with 40 Compton-
suppressed high purity Ge detectors and with the 80 BGO
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PHYSICAL REVIEW C 67, 061301R2003
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